The hardest metal bodies produced on a commercial scale for machining processes, drilling operations and other applications where wear resistance is necessary are cemented metal carbides. The most common of these is tungsten carbide which is sometimes combined with other carbides such as those of Ti, Cr, Ta and others. These materials are expensive to manufacture and are, where possible, frequently replaced in industry by high-speed steels and Stellites. Since the hard carbides are usually present in a softer matrix, their macro hardness is limited to about 68 on the Rockwell `C` scale.
Metallic borides are a group of materials which are very hard and comparable in this respect with carbides, nitrides and oxides and it is known to use boriding powders or pastes to obtain bodies with a very high surface hardness. Boriding powders are powders containing boron and particularly boron carbide which, when sintered with appropriate metals, forms the borides. The surface-hardening process is effected by a diffusion mechanism. This process involves diffusing boron into the surface of a metal body by bringing the boriding compound into close contact with the surface to be hardened and then allowing diffusion to take place whereby various metallic boride phases are formed. The boron usually diffuses to a depth of 500 microns although greater depths are attainable. The layer thus produced can have a hardness on the order of HR.sub.c 90 depending on the matrix and its treatment.